Derivative Instruments Fair Value of Derivative Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Commodity Swaps [Member]
Asset Derivative Instruments
Prepaid Expense	$ 34,985	$ 62,369
Other Assets	153,279	59,281
Total Asset	188,264	121,650
Liability Derivative Instruments
Other Accrued Liabilities	26,371	5,985
Other Liabilities	11,648	42,419
Total Liability	38,019	48,404
Basis Swap [Member]
Asset Derivative Instruments
Prepaid Expense	10,085	14,965
Other Assets	29,099	24,223
Total Asset	39,184	39,188
Liability Derivative Instruments
Other Accrued Liabilities	26,293	35,306
Other Liabilities	26,952	17,179
Total Liability	$ 53,245	$ 52,485